1. Field of the invention
The present invention relates to a semiconductor memory employing a block-write system, and more particularly to a semiconductor memory having a timing signal generator generating a block-write signal to prevent an internal address signal from changing during a block-write operation, and in which the semiconductor memory performs read/write operations synchronized with a clock signal supplied thereto.
2. Description of the Related Art
A semiconductor memory such as DRAM is often used for a video memory to storage video data which corresponds to characters or graphics to be displayed on a display device such as a CRT. To improve the write operation speed of the video memory, it has been proposed to use a block-write system which performs a block-write operation on a plurality (or "block") of memory cells simultaneously, because a plurality of memory cells having successive addresses in video memory often receive the same data. In video memory employing a block-write system, a plurality of column switches are activated simultaneously while a single word line is selected based on a row address at one write cycle, so that the same data is written simultaneously into a plurality of memory cells from a pair of I/O lines. Therefore the same data is written into a plurality of memory cells having successive addresses.
Since the block-write operation performs a writing operation simultaneously to a plurality of memory cells having mutually different column addresses, unlike the normal write operation, the operation requires more time for one operation compared with the normal write operation. Accordingly, if the cycle time of the normal write operation were shortened by increasing the frequency of the clock signal input to the memory, then a block-write operation might not be performed accurately because of insufficient processing time. In other words, a conventional semiconductor memory employing a block-write system cannot be used with a high-frequency clock signal, even if the normal write operation can be performed with such clock signal.